1. Field of the Invention
This invention relates to apparatus for a Coriolis mass flow rate meter and specifically such a meter that is suited for operation at relatively high temperatures, such as in excess of approximately 500.degree. F. (approximately 260.degree. C.).
2. Description of the Prior Art
Currently, Coriolis mass flow rate meters are finding increasing use in many applications as an accurate way to measure the mass flow rate of various process fluids.
Generally speaking, a Coriolis mass flow rate meter, such as that described in U.S. Pat. No. 4,491,025 (issued to J. E. Smith et al on Jan. 1, 1985), contains one or two parallel conduits, each typically being a U-shaped flow conduit or tube. Each flow conduit is driven, by a magnetic drive assembly, to oscillate about an axis to create a rotational frame of reference. For a U-shaped flow conduit, this axis can be termed the bending axis. As process fluid flows through each oscillating flow conduit, movement of the fluid produces reactionary Coriolis forces that are orthogonal to both the velocity of the fluid and the angular velocity of the conduit. These reactionary Coriolis forces cause each conduit to twist about a torsional axis that, for a U-shaped flow conduit, is normal to its bending axis. The amount of twist imparted to each conduit is related to the mass flow rate of the process fluid flowing therethrough. This twist is frequently measured using velocity signals obtained from magnetic velocity sensors that are mounted to one or both of the flow conduits in order to provide a complete velocity profile of the movement of each flow conduit with respect to either the other conduit or a fixed reference.
Coriolis meters known in the art, and as generally described above, utilize separate coils of wire for use within each magnetic velocity sensor and the magnetic drive assembly. Generally, a dual tube Coriolis meter has two magnetic velocity sensors situated in opposing positions on the sides of the two flow conduits and a single magnetic drive assembly frequently mounted to both flow conduits at respective opposing points thereon that are situated equidistant from corresponding ends of both conduits. In particular, each magnetic velocity sensor is typically fabricated with a sensing coil mounted to one of two flow conduits. A magnet that moves coaxially within the sensing coil is mounted to the other flow conduit. Whenever the magnet and sensing coil move in a differential sinusoidal pattern with respect to each other as dictated by respective differential sinusoidal conduit movement occurring thereat, the magnet induces a sinusoidal voltage in the sensing coil. The voltage produced by each sensing coil is then routed, through wiring, to an external electronic circuit which, in turn, determines the mass flow rate of the process fluid as a function of these two voltages. The drive assembly has a similar configuration to either sensing coil. In a dual tube meter, a drive coil that forms part of the drive assembly is mounted to one of the flow conduits with a magnet, also part of the drive assembly and adapted for coaxial movement within the coil, mounted at an opposing point on the other conduit. A sinusoidal voltage generated by the external electronic Circuit is applied, also via the wiring, to the drive coil. This voltage causes both the magnet and drive coil in the drive assembly to oscillate in a differential sinusoidal pattern thereby placing both flow conduits in opposing oscillatory motion about their respective bending axes.
Frequently, applications arise where accurate mass flow measurement is needed of a process fluid that flows at an elevated temperature, such as above 500.degree. F. (approximately 260.degree. C.). For example, one such application prevalent in the pulp and paper industry might involve the mass flow measurement of tall oil that flows, in a pulp and paper mill, at a temperature between 500.degree.-525.degree. F. (approximately 260.degree.-274.degree. C.). Because of this relatively high temperature range, an ordinary Coriolis meter is simply not suitable for this application. Generally speaking, most ordinary Coriolis meters known in the art are capable of operating at a temperature up to 400.degree. F. (approximately 204.degree. C.). Operation at temperatures that extend beyond 400.degree. F. becomes very problematical and at much higher temperatures, such as up to 800.degree. F. (approximately 427.degree. C.), essentially impossible for an ordinary Coriolis meter known in the art. This limitation arises for several reasons all due to the components used in such a meter.
In particular, while suitable alloys exist from which flow conduits can be fabricated for high temperature use, such as up to 800.degree. F., the same is generally not true for wiring and coils and associated components used in the meter.
First, wiring has insulation which carries certain temperature limits. Generally, the insulation that is used in ordinary light gauge coil wire and/or signal wiring can not withstand temperatures as high as approximately 430.degree. F. (approximately 220.degree. C.) before the insulation becomes plastic and melts. Clearly, if the wire used to make sensing and drive coils for use within a Coriolis meter were manufactured using such insulation, as is commonly done, then once the temperature of these coils exceeded approximately 430.degree. F., individual turns on the coil would likely short thereby, at the very least, injecting error into the performance of the meter. Moreover, any internal wiring which employed this insulation and ran between these coils and a suitable connector mounted on a case of the meter would also begin to melt thereby eventually permitting one or more of these wires to themselves contact the case of the meter which is generally grounded and as a result, in turn, cause the meter to malfunction.
Second, even assuming that a wiring insulation could be found that would withstand a high temperature in excess of 400.degree. F., the wiring that runs from the sensing and drive coils is often affixed to the flow conduits by a suitable tape, varnish or adhesive. These three materials often carry a temperature rating of up to approximately 430.degree. F. (approximately 220.degree. C.). As such, the tape, varnish and/or adhesive would melt as the temperature of the meter exceeded 430.degree. F. thereby causing the wiring to separate from the conduits and, in turn, inject error into the operation of the meter. At first blush, one would think that a wire coated with a high temperature ceramic insulation would be satisfactory. It may not be. Unfortunately, high temperature ceramic insulation tends to be brittle. As such, ceramic insulated wire could not be affixed to an oscillating flow conduit for any appreciable amount of time without the ceramic insulation eventually fatiguing and cracking which, in turn, would cause the wire itself to disadvantageously short against a grounded flow conduit thereby injecting error into the performance of the meter. Moreover, due to the brittle nature of ceramic insulation, ceramic coated wire can not be readily wound into a small diameter coil without the insulation breaking and possibly causing shorted turns.
Third, the bobbins on which the sensing and drive coils are wound are frequently plastic which itself has a rather low melting point. Thus, whenever the meter is heated, the coil bobbins would increasingly deform which would, in turn, inject error into meter performance.
Fourth, although various conductive materials exist that can be used for high temperature wire, most of these materials exhibit various drawbacks that render these materials unsuitable for use in a high temperature Coriolis meter. In particular, these materials include various nickel based alloys which tend to be magnetic. Unfortunately, if a sensing coil were to be wound with magnetic wire, the magnetic properties of the wire would interfere with the constant magnetic field generated within the velocity sensor and consequently inject measurement errors, particularly zero flow offset values, into the performance of the meter. Copper wire could not be used inasmuch as it exhibits grain growth at temperatures above 400.degree. F. which would cause creepage and eventual fatigue. Moreover, the resistance of most conductors increases with temperature. If wire manufactured from such a conductor were to be used in the drive coil, then as the temperature of the meter increases, additional power would disadvantageously need to be supplied to the drive coil in order to overcome the additional resistance of the drive coil.
For these reasons, the art turned to a solution exemplified by the teachings of U.S. Pat. No. 4,738,143 (issued to D. Cage et al on Apr. 19, 1988 and henceforth referred to as the '143 patent). This solution is aimed at providing a high temperature Coriolis mass flow rate meter in which the flow conduits themselves are thermally isolated from the wiring and coils. Specifically, this meter incorporates a thermally insulated partition in which the flow conduits are situated. Appropriate cutouts exist in the partition in order to permit the velocity sensors and drive coils to be mounted outside of the partition and on standoffs that emanate from the flow conduits. An active gas cooling purge line, typically using nitrogen gas flowing at a sufficient flow rate, is incorporated into the meter in order to maintain the velocity sensors and drive assembly at a relatively low temperature. An outer case surrounds the thermally insulated partition. Internal meter wiring is run in the space between the outer case and the thermal partition and is frequently wrapped around an internal purge tube.
While the meter disclosed in the '143 patent provides satisfactory operation at relatively high temperatures, it quickly became apparent that this meter also suffers various drawbacks that tend to limit its utility. First, owing to the incorporation of an internal thermal partition, the meter is complex, relatively expensive and difficult to build. Second, the meter requires that an active source of gas be connected to the purge line of the meter. The attendant increase in process plumbing necessitated by the purge line increases the complexity and cost of the installation of the meter, while the continual use of purge gas increases the cost associated with operating the meter. Third, proper operation of the meter necessitated that the cooling purge continually operate whenever the meter is at an elevated temperature. If the cooling purge fails for any reason while the meter remains at this temperature, then the coils and/or wiring would quickly fail. As such, a customer needs to exercise extreme vigilance over the status of the cooling purge if only to prevent the meter from becoming inadvertently destroyed. Unfortunately, routine monitoring of the cooling purge imposes an added maintenance burden on personnel at a customer location. In fact, the press of other more urgent matters might cause these personnel to not be sufficiently attentive and responsive to a failure in the cooling purge thereby inadvertently allowing the meter to overheat and be destroyed. Fourth, the purge line, particularly if the purge gas is set at an excessive flow rate, may cool the flow conduits within the meter to the point at which the high temperature process fluid flowing therethrough may disadvantageously freeze within either or both of the flow conduits. Fifth, heating blankets, when used, may also cause the meter to fail. Specifically, to prevent a high temperature process fluid from freezing, the lines, including any in-line ancillary metering equipment connected thereto through which that fluid will flow, would typically be wrapped in heating blankets which, in turn, would pump an adequate amount of heat into both the wrapped lines and in-line equipment. If such a blanket is wrapped around a Coriolis mass flow rate meter that has a purge line, then the additional heat generated thereby may overcome the cooling effect of the purge and disadvantageously cause the meter to heat to an excessively high temperature. As such, this additional heat may well cause an electrical component, such as a coil, situated within the meter to fail.
Therefore, a need exists in the art for a Coriolis mass flow rate meter that can reliably operate at temperatures in excess of 500.degree. F. and preferably as high as 800.degree. F. without the necessity of using a cooling purge. Such a meter should also be relatively simple, inexpensive and easy to build.